Microturbines and various types of fuel cells which are used in distributed generation are usually made in the form of co-generation systems including a heat exchanger (regenerator) which heats combustion air by utilizing the heat of combustion exhaust gas. The heat exchanger is constituted by corrugated fins, plates, and other parts made of stainless steel. It is desired that the stainless steel which is used have a high level of heat resistance and good weldability and workability.
Various fuels have been studied as a fuel source for supplying hydrogen in a fuel cell, such as natural gas, DME (dimethyl ether), kerosene, synthetic hydrocarbon fuels, and alcohol-based fuels. These fuels are reformed by a method such as steam reformation to generate hydrogen Due to the high operating temperature, it is desired that stainless steel which is used for the manufacture of such reformers also have a high level of heat resistance and good weldability and workability.
Gas which passes through the above-described heat exchangers and reformers contains from several percent to several tens of percent of steam and is at a high temperature. Such high temperature, humidified gas produces a severely corrosive environment compared to the atmosphere or to combustion exhaust gas from an automotive exhaust system.
In distributed generation, contrary to a conventional thermal power plant, starting and stopping of the operation of equipment occurs frequently, so the distributed generation is subjected to cyclic heating and cooling. As a result, a protective oxide scale primarily comprising Cr which is formed on the surface of stainless steel during heating develops thermal stresses during cooling and produces cracks, and eventually the scale peels off. In portions where the scale has peeled off, it is difficult to regenerate the protective scale, and frequently a non-protective oxide scale primarily comprising Fe is formed. As a result, a reduction in the wall thickness of the stainless steel is accelerated, and the service life of equipment is shortened. Furthermore, the peeled scale plugs up gas flow passages inside the equipment or gas flow passages connected to the exit side of the equipment. In addition, there is an extremely high danger of the peeled scale causing damage due to equipment by erosion.
Accordingly, it is desired that stainless steel used in applications such as heat exchangers or reformers for distributed generation have not only heat resistance, workability, and weldability, but also excellent resistance to scale peeling such that peeling of scale does not occur even when it undergoes repeated heating and cooling cycles in a highly corrosive, high temperature, humidified environment.
Up to now, many types of heat resistant Fe—Cr—Al ferritic stainless steels have been proposed as catalyst supports for cleaning equipment for automotive exhaust gas. However, ferritic stainless steels generally have poor workability and are difficult to weld, and they are difficult to apply to the above-described applications.
From in the past, austenitic stainless steels typified by SUS 304, SUS 316L, and SUS 310 have been much used for usual high temperature applications.
JP H07-188869 A1 discloses an inexpensive austenitic stainless steel which is superior with respect to resistance to oxidation at high temperatures, wear resistance, and creep properties and which has excellent weldability. This stainless steel comprises C: 0.05-0.15% (in this description, unless otherwise specified, percent means mass percent), Si: less than 1.0%, Mn: at most 2.0%, P: at most 0.04%, S: at most 0.01%, Cr: 20-30%, Ni: 10-15%, N: 0.10-0.30%, B: 0.0010-0.01%, La+Ce: 0.01-0.10%, Al: 0.01-0.20%, and a balance essentially of Fe and unavoidable impurities, with the Ni balance prescribed by {Ni+0.5Mn+30(C+N)−1.1 (Cr+1.5Si)+8.2} being in the range of −1.0% to +3%.
JP 2000-303150 A1 describes a stainless steel for direct diffusion bonding. This is a ferritic stainless steel comprising C: at most 0.08%, Si: 0.01-2%, Mn: 0.05-1.5%, P: at most 0.05%, S: at most 0.01%, Al: 0.005-0.1%, Cr: 13-32%, Ni: 0.01-4%, Mo: 0.1-6%, Ti: at most 0.05%, and a balance of Fe and unavoidable impurities, or an austenitic stainless steel comprising C: at most 0.08%, Si: 0.01-2%, Mn: 0.05-1.5%, P: at most 0.05%, S: at most 0.01%, Al: 0.005-0.1%, Cr: 13-25%, Ni: 7-15%, Si+Mo: at least 1.5%, Mo: at most 6%, Ti: at most 0.05%, and a balance of Fe and unavoidable impurities. This austenitic stainless steel is described as being easy to roll and having excellent workability.
However, the austenitic stainless steels described in the above-described publications do not take into consideration the above-mentioned resistance to scale peeling.
JP H11-279714 A1 discloses an austenitic stainless steel having improved resistance to scale peeling under conditions in which a temperature gradient is present in the material during heating and cooling cycles. This stainless steel comprises C: 0.01-0.15%, Si: 0.5-5%, Mn: 0.2-2%, P: at most 0.04%, S: at most 0.02%, Ni: 12-22%, Cr: 17-26%, Al: 0.01-5%, N: 0.02-0.4%, and a balance of Fe and unavoidable impurities, with the added amount of alloying elements, the maximum heating temperature TK, and the temperature gradient α (° C./mm) satisfying a prescribed relationship.
That austenitic stainless steel utilizes a phenomenon in which oxides of Si or Al which are concentrated at the boundary of scale or metallic Ni alleviate strains due to expansion and contraction applied to scale when a temperature gradient is present in a material to improve the resistance to scale peeling. Therefore, the contents of Ni, Si, and Al are important, and the contents of Cr, Ni, Si, and Al in the steel are prescribed by the maximum temperature T and the temperature gradient α in the material. However, the resistance to scale peeling of that stainless steel is not of a level which can satisfy the resistance to scale peeling needed in severe humidified gas corrosive environments containing from several percent to several tens of percent of steam. Furthermore, consideration is not given to regeneration after peeling of scale, and it is necessary to improve its stability with respect to long-term properties.
In JP 2003-171745 A1, the present inventors proposed an austenitic stainless steel plate containing C: 0.01-0.10%, Si: 0.01-1.0%, Cr; 19-26%, Ni: 10-35%, a total of 0.005-0.10% of at least one REM (rare earth metal), at least 0.01% of Mn satisfying {Mn≦2.8×REM−0.025×Ni+0.95}, and a balance of Fe and unavoidable impurities and having a thickness of at most 1.0 mm. That invention suppresses the growth rate of Cr2O3 by controlling the Mn content in accordance with the Ni content and the REM content in order to solve the phenomenon of “burnout” caused by abnormal oxidation in a thin steel plate such as a corrugated fin of distributed generation equipment.
In JP 2004-83976 A1, the present inventors farther proposed an austenitic stainless steel plate having high temperature oxidation resistance and containing C: 0.01-0.10%, Si: at most 1.0%, Cr: 23.0-27.0%, Ni: 17.0-23.0%, a total of 0.005-0.10% of at least one rare earth element, a Mn content of at most 2.0% satisfying the relationship {Mn≦0.05×Cr−0.20×plate thickness−0.55}, and a balance of Fe and unavoidable impurities and having a thickness of at most 0.5 mm. That invention improves the high temperature strength of Cr2O3 scale by controlling the relationship between the Mn content, the Cr content, and the plate thickness based on the finding that “accelerated oxidation” which develops in a steel plate with a thickness of at most 0.5 mm is caused by deformation occurring when the strength is overcome by the stress which develops in scale which forms on the surface of steel.
The austenitic stainless steel plates which are proposed by above-mentioned JP 2003-171745 A1 and JP 2004-83976 A1 can solve the problems of burnout and accelerated oxidation which develop in thin stainless steel plates. However, there is no recognition in those publications of peeling of scale which is a problem in the above-described heating and cooling cycles, and there is no suggestion concerning a method of solving that problem.